1. Field of the Invention
This invention relates to the generation of seismic energy in water by the use of seismic sources of the pressurized gas abrupt releasing type and more particularly to apparatus for permitting maximum acoustic output at the earliest possible time.
2. Description of the Prior Art
In marine seismic exploration, a source of acoustic energy is released into the water every few seconds to obtain appropriate acoustic waves that propagate into the earth's surface. These waves are reflected at interfaces of the subsurface formations and propagated back to instruments where transducers convert the acoustic waves to electronic signals which are recorded and later processed into a record section for interpretation of the subsurface formations.
In recent times, the major marine seismic energy source has been the air gun. These air guns release high pressure air, typically 2,000 psi to 6,000 psi into the water to create the desired acoustic wave.
Current air guns each normally comprise an annular housing that contains means for discharging compressed air through exhaust ports in the housing. Compressed air is stored within the housing in a firing chamber. The only moving component (except for the solenoid triggering device) is a shuttle, which when raised, permits air to escape from the firing chamber through the ports in the main housing into the surrounding water. The size of the gun is determined by the firing chamber volume selected. By having a constant source of compressed air through an inlet passage in the housing, the upper chamber containing the shuttle is filled and forces the shuttle into a sealed position, closing off all exhaust ports from the firing chamber. By using a solenoid valve to allow air flow underneath the shuttle flange, thus forcing the shuttle upward and causing an uequal pressure on the shuttle, the shuttle is accelerated in the upward direction exposing the chamber exhaust ports and allowing compressed air to escape into the surrounding water. When the shuttle is in the down, or pre-fire position, the air gun is charged and ready for firing. When fired, this air gun allows 80% to 90% of the air in the firing chamber to be exhausted into the water.
A more recent air gun is described and claimed in U.S. Pat. No. 4,230,201, assigned to the assignee of this invention. This air gun has a shuttle control that closes the exhaust ports before all of the compressed air in the firing chamber of the air gun is exhausted. By preventing further discharge of air, secondary pulses are greatly reduced and the supply of air requirement is reduced.
Other prior art air guns include internal and external sleeve-type shuttle air guns. These devices are described and claimed in U.S. Pat. Nos. 4,225,009 and 4,219,097, respectively, assigned to the assignee of this invention. In these and other prior art sleeve type air guns, the port area is much smaller than in the sleeve type air gun of this invention. Furthermore, in the prior art, when the shuttle is released, there is no further control, the shuttle returning to its pre-fire condition only by virtue of air pressure dropping to some predetermined level.